This invention relates to a glass fiber nonwoven fabric suitable for use as an insulation-reinforcing material for a glass fiber-reinforced laminate, such as a double-copper-clad, printed wiring board, a multilayered printed wiring board or the like, and also relates to a printed wiring board in which the above glass fiber nonwoven fabric is used.
Moreover, this invention relates to a highly flat glass fiber to be used in the production of a glass nonwoven fabric or the like and to a nozzle chip for spinning a highly flat glass fiber, a method for placing nozzle chips and apparatus for producing a glass fiber.
Recently, a glass cloth has been broadly used as an insulation-reinforcing material for a printed wiring board and partly, a glass fiber nonwoven fabric (also named glass paper) has also been used. For example, in the field of low price goods, there is used a double-copper-clad, printed wiring board which is called Composite CEM-3 in which a glass fiber nonwoven fabric is used as a core material and a glass cloth is used on both sides. The glass fiber nonwoven fabric used here is one obtained by subjecting a dispersion in water of glass fibers having a circular section of 3 to 10 xcexcm in diameter and having a length of about 3 to 25 mm to papermaking by a paper machine, and usually, it has a thickness of 100 to 700 xcexcm and an apparent amount of about 25 to 100 g/m2.
However, the bulk density of the conventional glass fiber nonwoven fabric is low, so that in a printed wiring board in which the glass fiber nonwoven fabric is used, the amount of a resin contained in the printed wiring board becomes large, and there have been such problems that as compared with a glass-epoxy resin printed wiring board in which the surfaces and the layer of the whole of the core material are composed of glass cloth, the coefficient of thermal expansion of the core material portion is large and the reliability of the through-hole plating portion is inferior.
Furthermore, since the conventional glass fiber nonwoven fabric is formed by subjecting circular cross-section glass fibers to papermaking, substantially no entanglement is present between fibers, and hence, a large amount of a binder (for example, 10 to 13% by weight) is applied for imparting the necessary tensile strength to the glass fiber nonwoven fabric. However, surface active agents to be contained in an emulsion type adhesive used as the binder have such characteristics as to lower the binding power between glass fiber surface and matrix resin, and hence, there are such issues that with an increase of the amount of the binder, the lowering of the binding power between glass fiber surface and matrix resin becomes large, the hot water resistance and heat resistance are deteriorated, and the insulation resistance after pressure cooker test is deteriorated.
In addition, generally, in the course of producing a prepreg, such a so-called sink mark is caused that the solvent in the resin is removed, the volume of the resin portion is reduced, and the resin is moved from the surface to the interior, and in the molding step, too, the volume shrinkage of the resin is caused after hot pressing under pressure. However, when a prepreg is produced using a glass fiber nonwoven fabric on the surface, a copper foil is laminated to the surface thereof and the resultant is subjected to press molding, the bulk density of the glass fiber nonwoven fabric is low and the amount of the resin becomes large, so that the sinking phenomenon is marked and the volume shrinkage of the resin is largely caused. Therefore, irregularities approximate to 10 xcexcm are formed on the resin face on the surface of the laminate produced and similar irregularities appear even on the copper foil laminated thereto. Hence, when a circuit is formed by partly removing the copper foil by etching or the like, the copper foil in the concave portions tends to remain (is hardly removed) and the copper foil in the convex portions is easily removed, so that the fine circuit is disconnected, or erroneously connected through the remaining copper foil. Moreover, the adhesion of copper foil becomes uneven and hence similar problems such as disconnection and the like are caused. Therefore, the conventional glass fiber nonwoven fabric cannot be used on the surface to which a copper foil is to be laminated.
Thus, the conventional glass fiber nonwoven fabric is inferior to glass cloth in the characteristics when used in a printed wiring board, and cannot be used on the surface. Therefore, it must be used in the intermediate layer of a double-copper-clad, printed wiring board.
Therefore, an attempt for improving the characteristic of the conventional glass fiber nonwoven fabric to enable it to be used in the printed wiring board in place of the glass cloth has been made using glass fibers having a modified cross section and a proposal is disclosed in JP-B-7(1995)-122,235, JP-A-6(1994)-257,042, JP-A-8(1996)-127,994 and the like. In these official publications, there are proposed glass fiber nonwoven fabrics prepared using flat glass fibers having a flat sectional shape such as elliptic shape, cocoon shape, capsule shape or the like, and the publications describe that these can make the bulk density large and simultaneously can increase the tensile strength. In addition, JP-A-6(1994)-257,042 describes that by using the flat cross-section glass fibers, the amount of the binder can be reduced (down to about 3% by weight).
Surely, it is possible to increase the bulk density of the nonwoven fabric to some extent by use of the glass fibers having a fiat sectional shape and to make the irregularities on the surface as small as about 4 xcexcm when the glass fiber nonwoven fabric on the surface of a laminate is used. However, for the glass fiber nonwoven fabric to be used in place of the glass cloth, a further improvement in characteristics is desired, particularly a high content and an improvement in surface smoothness have been desired.
Recently, as, for example, electronic equipment has been miniaturized and the performance thereof has been made high, there has been used a laminate of a plurality of circuit boards which is called a multilayered board (or a multilayered printed wiring board). As a result, it is desired that the width of the line constituting the circuit is made still smaller to closely place the same. For responding to such demands, it is desired that the copper foil is made as thin as possible (for example, about 12 xcexcm or less); however, when the copper foil is made thin, it follows that the copper foil is more greatly affected by the irregularities on the laminate surface to which the copper foil is to be laminated. Therefore, it is required to further enhance the surface smoothness of the laminate. However, even in laminates in which the conventional glass cloth is used, the limit of the surface smoothness is about 3 xcexcm, and no higher smoothness can be obtained. In this respect, there has been a limit in the reduction of copper foil thickness. Accordingly, there has been desired development of a reinforcing material by which the surface smoothness of the laminate can be improved as compared with the case where a glass cloth is used, and development of a glass fiber nonwoven fabric meeting the said desire has been desired.
Furthermore, when a glass fiber nonwoven fabric is used in the multilayered printed wiring board, it is desirable that the above glass fiber nonwoven fabric is as thin as possible, and one having an apparent amount of about 15 to 40 g/m2 is required. However, with such a thin glass fiber nonwoven fabric, there has also been such a problem that the amount of the binder cannot be made so small for maintaining the necessary strength. For example, JP-A-6(1994)-257,042 describes that the amount of the binder can be made 3% by weight; however, this is a case where the nonwoven fabric is thick, and according to the present inventors"" duplication, as shown in Comparative Examples 1 and 2 which are described hereinafter, no prepreg was able to be prepared when the apparent amount was 20 g/m2 and the amount of the binder was 6% by weight, and an amount of 10% by weight was required for maintaining the necessary strength for preparing a prepreg.
This invention has been made based on such demands and aims at not only increasing the bulk density of the nonwoven fabric, but also providing a thin glass fiber nonwoven fabric having the necessary strength even when the amount of a binder is decreased, which makes it possible to increase the glass fiber content in a laminate in which the above glass fiber nonwoven fabric is used as a reinforcing material and to enhance the surface smoothness thereof, and also aims at providing a printed circuit board composed of a laminate in which the above glass fiber nonwoven fabric is used as a reinforcing material.
Moreover, this invention aims at providing such a highly flat glass fiber that the flatness ratio represented by the longest major axis/longest minor axis rectangular to the longest major axis of the section of flat glass fiber is 2.0 to 10 and the ratio of the sectional area of flat glass fiber to the area of a rectangle circumscribed about the section of flat glass fiber (the ratio is hereinafter referred to as the packing fraction) is at least 85%, preferably 90 to 98%.
Furthermore, this invention aims at providing a nozzle chip for spinning a glass fiber which enables the above-mentioned flat glass fiber to be produced with stable quality and with good productivity and a process or placing nozzle chips therefor.
For achieving the above-mentioned objects, the present inventors have made various examinations on the sectional shape of a glass fiber to be used in a glass fiber nonwoven fabric and have consequently found that when a nonwoven fabric is prepared by a papermaking method using such a highly flat glass fiber whose section has a flat shape that the flatness ratio of said section is 2.0 to 10, preferably 3.1 to 8 and the packing fraction is at least 85%, preferably at least 90% and more preferably 93 to 98%, there can be prepared a thin nonwoven fabric in which almost all flat glass fibers are piled one on another with the flat side down and simultaneously the area of contact of the piled flat glass fibers with one another becomes large, and even when the amount of a binder is as very small as about 3 to 8% by weight, the nonwoven fabric has the strength necessary for handling; that the nonwoven fabric obtained has a high bulk density; and that the surface smoothness of a laminate prepared with the nonwoven fabric is very good, whereby this invention has been accomplished.
That is to say, the glass fiber for forming the glass fiber nonwoven fabric in this invention is a highly flat glass fiber whose section has a flat shape (abbreviated hereinafter merely as xe2x80x9cthe flat glass fiberxe2x80x9d in some cases), the flatness ratio of the section is 2.0 to 10, preferably 3.1 to 8, and the packing fraction thereof is at least 85%, preferably at least 90% and more preferably 93 to 98%. And, the glass fiber nonwoven fabric of this invention is such that a flat glass fiber having the above-mentioned specific flatness ratio and packing fraction and having a reduced fiber diameter of 5 to 17 xcexcm is used in a proportion of at least 90% by weight based on the total nonwoven fabric weight excluding the weight of the binder and the amount of the binder is adjusted to 3 to 8% by weight.
In the glass fiber nonwoven fabric of this invention, the highly flat glass fiber having the above-mentioned construction is used, and therefore, when a nonwoven fabric is formed by subjecting the above flat glass fiber to papermaking, almost all flat glass fibers become piled one on another with the flat side down, hence, the bulk density becomes high and simultaneously the area of contact of the piled flat glass fibers with one another is increased, the binding with a small amount of a binder can impart a large strength to the nonwoven fabric, and the smoothness of the surface of the nonwoven fabric per se is good. Thus, this invention can provide a glass fiber nonwoven fabric which is thin and has a large bulk density and a high surface smoothness and in which the amount of a binder used is small, and a laminate prepared using the above glass fiber nonwoven fabric has a high glass fiber content, a good dimensional stability, excellent water resistance and heat resistance and further has an excellent surface smoothness. Therefore, by using it as an insulation-reinforcing material for a printed wiring board in place of the glass cloth, a high performance printed wiring board can be produced.